template instantiation with multiple template inheritance

问题

What are the rules for template instantiation when we pass a (multi)derived class to a template function expecting base class? For example:

#include <iostream>

template <int x>
struct C {};

struct D : C<0>, C<1> {};

template <int x>
void f (const C<x> &y)  { std::cout << x << "\n"; }

int main ()
{
  f (D ());
}

MSVC 2015 prints 0, clang 3.8 - 1 and gcc 6.2 gives compiler error (Demo). And even if you SFINAE-away all overloads except one, the result will still be different:

#include <iostream>

template <int x> struct C {};

template<>
struct C<0> { using type = void; };

struct D : C<0>, C<1> {};

template <int x, typename = typename C<x>::type>
void f (const C<x> &y)  { std::cout << x << "\n"; }

int main ()
{
  f (D ());
}

Now it compiles only with MSVC, and if you swap C<0> and C<1> only clang will compile it. The problem is that MSVC only tries to instantiate first base, clang - last and gcc prints error too early. Which compiler is right?

回答1:

gcc 5.4:

/tmp/gcc-explorer-compiler11685-58-1h67lnf/example.cpp: In function 'int main()':
13 : error: no matching function for call to 'f(D)'
f (D ());
^
9 : note: candidate: template<int x> void f(const C<x>&)
void f (const C<x> &y) { std::cout << x << "\n"; }
^
9 : note: template argument deduction/substitution failed:
13 : note: 'const C<x>' is an ambiguous base class of 'D'
f (D ());
^
Compilation failed

Which seems to me to be the correct result, since C<0> and C<1> are equally specialised.

Same result for gcc 6.2

clang 3.8.1 compiles it, which in my view is a compiler bug.

update:

I don't know the actual use case but I was wonder whether this might work for you:

#include <utility>
#include <iostream>

template<class T>
struct has_type
{
    template<class U> static auto test(U*) -> decltype(typename U::type{}, std::true_type());
    static auto test(...) -> decltype(std::false_type());
    using type = decltype(test((T*)0));
    static const auto value = type::value;
};

template <int x> struct C {};

template<>
struct C<0> { using type = int; };

template<int...xs>
struct enumerates_C : C<xs>...
{
};

struct D : enumerates_C<0, 1> {};

template<int x, std::enable_if_t<has_type<C<x>>::value>* = nullptr>
void f_impl(const C<x>& y)
{
    std::cout << x << "\n";
}

template<int x, std::enable_if_t<not has_type<C<x>>::value>* = nullptr>
void f_impl(const C<x>& y)
{
    // do nothing
}

template <int...xs>
void f (const enumerates_C<xs...> &y)
{
    using expand = int[];
    void(expand { 0,
        (f_impl(static_cast<C<xs> const &>(y)),0)...
    });
}

int main ()
{
    f (D ());
}

expected output (tested on apple clang):

来源：https://stackoverflow.com/questions/39331632/template-instantiation-with-multiple-template-inheritance

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c++

templates

language-lawyer

overload-resolution